Hexose oxidase (D-hexose: O2-oxidoreductase, EC 1.1.3.5) is an enzyme which in the presence of oxygen is capable of oxidizing D-glucose and several other reducing sugars including maltose, lactose and cellobiose to their corresponding lactones with subsequent hydrolysis to the respective aldobionic acids. Accordingly, hexose oxidase differ from another oxido-reductase, glucose oxidase which can only convert D-glucose in that this enzyme can utilize a broader range of sugar substrates. The oxidation catalyzed by hexose oxidase can e.g. be illustrated as follows:D-Glucose+O2--->δ-D-gluconolactone+H2O2, orD-Galactose+O2--->γ-D-galactogalactone+H2O2
Up till now, hexose oxidase (in the following also referred to as HOX) has been provided by isolating the enzyme from several red algal species such as Iridophycus flaccidum (Bean and Hassid, 1956) and Chondrus crispus (Sullivan et al. 1973). Additionally, the algal species Euthora cristata has been shown to produce hexose oxidase.
It has been reported that hexose oxidase isolated from these natural sources may be of potential use in the manufacturing of certain food products. Thus, hexose oxidase isolated from Iridophycus flaccidum has been shown to be capable of converting lactose in milk with the production of the corresponding aldobionic acid and has been shown to be of potential interest as an acidifying agent in milk, e.g. to replace acidifying microbial cultures for that purpose (Rand, 1972). In that respect, hexose oxidase has been mentioned as a more interesting enzyme than glucose oxidase, since this latter enzyme can only be utilized in milk or food products not containing glucose with the concomitant addition of glucose or, in the case of a milk product, the lactose-degrading enzyme lactase, whereby the lactose is degraded to glucose and galactose. Even if glucose in this manner will become available as a substrate for the glucose oxidase, it is obvious that only 50% of the end products of lactase can be utilized as substrate by the glucose oxidase, and accordingly glucose oxidase is not an efficient acidifying agent in natural milk or dairy products.
The capability of oxygen oxidoreductases including that of hexose oxidase to generate hydrogen peroxide, which has an antimicrobial effect, has been utilized to improve the storage stability of certain food products including cheese, butter and fruit juice as it is disclosed in JP-B-73/016612. It has also been suggested that oxidoreductases may be potentially useful as oxygen scavengers or antioxidants in food products.
Within the bakery and milling industries it is known to use oxidizing agents such as e.g. iodates, peroxides, ascorbic acid, K-bromate or azodicarbonamide for improving the baking performance of flour to achieve a dough with improved stretchability and thus having a desirable strength and stability. The mechanism behind this effect of oxidizing agents is that the flour proteins, such as e.g. gluten in wheat flour contains thiol groups which, when they become oxidized, form disulphide bonds whereby the protein forms a more stable matrix resulting in a better dough quality and improvements of the volume and crumb structure of the baked products.
However, such use of several of the currently available oxidizing agents are objected to by consumers or is not permitted by regulatory bodies and accordingly, it has been attempted to find alternatives to these conventional flour and dough additives and the prior art has suggested the use of glucose oxidase for the above purpose. Thus, U.S. Pat. No. 2,783,150 discloses the addition of glucose oxidase to flour to improve the rheological characteristics of dough. CA 2,012,723 discloses bread improving agents comprising cellulolytic enzymes and glucose oxidase and JP-A-084848 suggests the use of a bread improving composition comprising glucose oxidase and lipase.
However, the use of glucose oxidase as a dough and bread improving additive has the limitation that this enzyme requires the presence of glucose as substrate in order to be effective in a dough system and generally, the glucose content in cereal flours is low. Thus, in wheat flour glucose is present in an amount which is in the range of 0-0.4% w/w, i.e. flours may not contain any glucose at all. Therefore, the absence or low content of glucose in doughs will be a limiting factor for the use of glucose oxidase as a dough improving agent. In contrast, the content of maltose is significantly higher already in the freshly prepared dough and further maltose is formed in the dough due to the activity of β-amylase either being inherently present in the flour or being added.
The current source of hexose oxidase is crude or partially purified enzyme preparations isolated by extraction from the above natively occurring marine algal species. However, since the amount of hexose oxidase in algae is low, it is evident that a production of the enzyme in this manner is too tedious and costly to warrant a cost effective commercial production of the enzyme from these natural sources. Furthermore, the provision of a sufficiently pure enzyme product at a cost effective level is not readily achievable in this manner.
A considerable industrial need therefore exists to provide an alternative and more cost effective source of this industrially valuable enzyme without being dependent on a natural source and also to provide the enzyme in a pure form, i.e. without any contaminating enzyme activities or any other undesirable contaminating substances including undesirable algal pigments and environmental pollutants which may be present in the marine areas where the hexose oxidase-producing algal species grow.
Furthermore, the industrial availability of a food grade quality of hexose oxidase in sufficient amounts and at cost effective prices will undoubtedly open up for new applications of that enzyme not only in the food industry, but also in other industrial areas as it will be discussed in the following. One example of such a novel application of the recombinant hexose oxidase in the food industry is the use hereof as a dough improving agent, another example being the use of hexose oxidase active polypeptide or a recombinant organism producing the polypeptide in the manufacturing of lactones.